This invention relates generally to systems for inspecting printed circuit boards, and more particularly, to an improved system for analyzing successive viewing fields at an increased rate.
As is well known to persons skilled in the art, a printed circuit board is used for mounting and electrically interconnecting electrical components in a predetermined manner. To the extent possible, such printed circuit boards are constructed mechanically, using automated assembly machines which operate to reduce the often prohibitive costs of manually assembling a printed circuit board. While reducing overall costs, such automated assembly techniques have been found to give rise to a certain limited number of assembly defects such as incorrect insertions of components, and their leads or pins, as well as defects in the soldering procedures which then follow.
Originally, steps were taken to locate assembly errors of this general type through a visual inspection of each printed circuit board at a desired stage of the manufacturing process, by human operators using the naked eye, or possibly a stereo microscope or the like. However, since this procedure was found to be extremely tedious and inaccurate, as well as a relatively expensive process, steps were taken to develop automated systems for inspecting printed circuit boards, to replace such visual inspections.
Examples of devices of this general type are the Model 5511, Model 5512, Model 5515, Model 5516 and Model 5517 Printed Circuit Board Inspection Systems which are manufactured by Control Automation Incorporated of Princeton, N.J. These inspection devices generally employ a series of cameras which are mounted within a fixture (an inspection head) adapted for controlled movement relative to a printed circuit board. The inspection head is either sequentially advanced to successive viewing fields (typically one inch by one inch) established along the surface of the printed circuit board then under inspection, or continuously advanced along the surface of the printed circuit board, to acquire images for microprocessor analysis. Any detected defects are in turn reported to the operator, for appropriate correction.
Such devices operate to enhance the accuracy of the inspection process by providing an inspection head which incorporates a series of four angled, orthogonally placed cameras, operated in conjunction with a selectively controllable light source. Through selective control of this series of cameras, and the associated light source, a variety of testing procedures are enabled including a verification of the placement of components (and their leads or pins), both before and after the soldering procedure, as well as a verification of the solder connections which are made.
Initially, such inspections were accomplished by sequentially advancing the inspection head (or the printed circuit board) through successive viewing fields, and by selectively activating the series of cameras and their associated lighting to acquire images for inspection purposes. Later, primarily in order to increase the rate at which circuit board inspections could be accomplished, such inspections were accomplished by continuously advancing the inspection head (or the printed circuit board) through its successive viewing fields, and by selectively strobing the associated lighting system to acquire images for inspection purposes. However, even this enhancement was found to have certain limitations in terms of the rate at which printed circuit boards could be inspected, primarily resulting from limitations associated with the video systems which were employed.
In overall operation, the continuous inspection of a printed circuit board involves scanning of the board in a given direction and at a rate which allows three successive functions to take place including scanning, frame storage and processing. Such techniques, which are often referred to as "pipelining", are primarily limited by the rate at which video images are acquired by the video cameras during the scanning portions of this procedure. The primary reason for this is that conventional video formatting (for the cameras and for display on a monitor) operates within a specified bandwidth, to develop images which are "interlaced" so that changes in the image from frame to frame are less noticeable to the human eye. To this end, the series of lines which conventionally comprise a video image are operated upon in alternating fashion so that a first series of alternating lines (generally referred to as the "odd field") is acquired and/or processed, followed by the acquisition and/or processing of a second series of alternating, interlaced lines (referred to as the "even field"). Resulting from this, the successive (odd/even) fields of a video image are combined to develop a complete frame for subsequent processing, all of which must be accomplished within the operative bandwidth for the system.
Consequently, a period of 33.3 milliseconds is generally required to develop a single frame of video information, for subsequent storage and processing (allowing 16.7 milliseconds for processing the odd and even fields, respectively). The rate at which printed circuit boards can be inspected by existing equipment is correspondingly limited by the rate at which video images can be acquired (i.e., at 33.3 millisecond intervals). For a conventional viewing field (typically one inch by one inch), this limits the rate at which circuit boards can be inspected to approximately 15 inches per second. The reason for this is that the views which are to be acquired by the series of cameras associated with the inspection apparatus are overlapped by 50% (with paired cameras utilizing different lighting modes). As a consequence of this, the inspection head must travel one-half inch (for each pair of cameras) in 33.3 milliseconds, or 15 inches per second.